This invention relates to an engine control system and, more particularly, to an engine control system the purpose whereof is to reduce car body vibration.
Conventionally, when a vehicle in which power is transferred without the intervention of a fluid coupling or the like is subjected to comparatively sudden acceleration, the vehicle experiences front-and-rear vibration in which the car body wobbles. The reason for this is that a change in engine torque produces a source of vibration which is increased by resonance of the drive system. Though attempts have been made to solve this problem by limiting the change in engine torque or rationalizing the rigidity of the drive system, the results have not been entirely satisfactory. One reason for this is that resonance itself cannot be suppressed by limiting a change in torque. Another is that increasing drive system rigidity not only has little effect upon reducing vibration but also invites a increase in engine weight and poorer fuel economy.
Accordingly, systems have been proposed for suppressing front-and-rear vibration of a car body by varying engine torque at the time of acceleration without inviting the inconveniences of increased engine weight and the like. For example, see Japanese Patent Application Laid-Open (KOKAI) Nos. 59-165865, 58-48738, 59-113269 (U.S. Pat. No. 4,527,523) and 60-6071 (U.S.Pat. No. 4,498438), as well as U.S. Pat. No. 4,345,559. For instance, in the system of Japanese Patent Application Laid-Open No. 58-48738, a change in engine rotational speed is sensed and it is so arranged that engine torque takes on a reciprocal characteristic with respect to the change in rotational speed. More specifically, the output of an engine rpm sensor is passed through a low-pass filter adapted to extract the vibration frequency component of drive system torsion, thereby detecting a rotational variation. If this value exceeds a threshold value, the system effects control of ignition advance angle commensurate with the rotational variation.
The inventors, who have taken notice of the fact that car body vibration is not actually suppressed even when torque control is performed in accordance with the prior art set forth above, have discovered that the cause of this problem resides in the fact that the timing of the change in engine torque brought about by engine torque control does not match the actual timing of car body vibration. In other words, though the prior art mentioned above primarily focuses on matching the period of car body vibration and the control period of torque control, car body vibration cannot be suppressed because of a phase difference between the timing of torque control and car body vibration.
In pursuing the cause of the aforementioned mismatch between the timing of the change in engine torque and the actual timing of car body vibration, namely the cause of the phase difference between torque control timing and car body vibration, the inventors have found that the cause resides in the fact that all of the prior-art systems depend upon an ideal control model of an engine. So far as reliance is placed upon an ideal model, a phase difference between torque control timing and car body vibration does not constitute a problem. In actuality, however, a variety of "control delays" occur.
Control delays arise for certain reasons. For instance, the control circuit (primarily a digital microcomputer) actually used in engine control is capable of acquiring only external data which is discrete in terms of time. For example, in a case where a control unit obtains data indicative of engine rotational speed, the control unit is adapted to perform computations based on the period at which at least two pulses are generated by a sensor provided on a crankshaft. However, since the result of a computation itself involves a time-wise lag element (which lag corresponds to the pulse generation interval), what is obtained is not realtime engine rotational speed, namely engine speed from one instant to the next. That is to say, at the moment engine rotational speed is acquired, the "control delay" has already occurred.
Further, the output of the engine rotational speed sensor contains noise In order to cancel out this error component, the usual practice is to average, over several revolutions, the engine rotational speed obtained as described above. However, the fact that this average value is actually delayed in time because of the time-shared control of the digital microcomputer is also a cause of a control delay.
Accordingly, even if the prior art succeeds in matching the control period of torque control with the period of car body vibration, the control delays that inevitably arise in an electronic control system are neglected in the individual control periods of torque control. As a result, torque control is carried out based on past data corresponding to control delay. In addition, torque fluctuation and the natural vibration of the drive system resonate in certain cases, so that suppression of the front-and-rear vibration of the car body is delayed.
Thus, the torque control timing delay that arises in the electronic control system of the vehicle engine is one reason for the poor reduction in car body vibration in the prior art.
The inventors have pursued, from another standpoint, the cause of the aforementioned mismatch between the timing of the change in engine torque and the actual timing of car body vibration, namely the cause of the phase difference between torque control timing and car body vibration. As a result, the inventors have made an important discovery. Specifically, when engine output is transmitted to the wheels of the vehicle through the drive system inclusive of the transmission, thereby causing the car body to vibrate, drive system torsion results in a time-wise delay between the change in the rotational speed of the engine and the vibration of the car body. Though substantially constant while car body vibration is in a state of resonance, this delay time differs according to the operating state of the engine at the beginning of acceleration (particularly up to the time of the initial peak of vibration at the beginning of acceleration). The reason for this is that the aforementioned torsion in the drive system at the transition from cruising at a steady velocity to acceleration differs from that at the transition from a decelerating to an accelerating traveling state, by way of example. Accordingly, even if the control period of torque control is made to coincide with the period of car body vibration, the fluctuation in torque and the natural vibration of the drive system will resonate in certain cases unless the two timings match at the beginning of acceleration. The result will be a delay in the suppression of front-and-rear vibration of the car body.
Thus, the fact that the state of drive system torsion differs depending upon a variation in the operating state of the vehicle at acceleration is one reason for non-coincidence between the timing of torque control and the timing of car body vibration.